The present invention relates to high resolution intravascular imaging and more particularly to intravascular ultrasound imaging and techniques for enhancing image quality.
In intraluminal or intravascular ultrasound (also referred to as "IVUS") imaging, the production of high resolution images of vessel wall structures requires imaging at high ultrasound frequencies. IVUS imaging systems may utilize electronic scanners or mechanical scanners. IVUS systems utilizing electronic scanning typically include in the distal end of a catheter an array of ultrasound transducers which are sequentially excited so as to electronically scan an ultrasonic beam. IVUS systems utilizing mechanical scanning (one example of such a system being shown in FIG. 1) may use a single rotating transducer 1 in the distal end of a catheter 3 that enters the blood vessel 20, with a drive shaft 5 coupling the transducer 1 to a motor (not shown) coupled to the catheter 3 at its proximal end. IVUS systems using mechanical scanning have wider applications, mainly due to the smaller size of the mechanical scanner in comparison with electronic scanner, that advantageously allow the system to be used for smaller blood vessels as well as larger blood vessels.
The present invention relates to IVUS imaging systems with mechanical scanning. In these types of IVUS systems, an ultrasonic unidirectional exciter/detector (e.g., transducer) within a catheter probe positioned within a blood vessel is used to acquire signal data from echoes of the emitted ultrasonic energy off the interior of the blood vessel. Specifically, vectors are created by directing focused ultrasonic pressure waves 2 radially from a transducer in a catheter and collecting echoes 4 at the same transducer from the target area, as seen in FIG. 1. In an exemplary IVUS system with mechanical scanning, the transducer is mechanically rotated at a uniform speed with multiple firings of ultrasonic excitation in order to obtain a plurality of equally spaced radial vectors from the collected echoes. The plurality of radial vectors from the rotated transducer comprises an image frame. A signal processor then performs image processing (e.g., stabilization of a moving image, temporal filtering for blood speckle, and other image enhancement techniques) on the acquired data in order to provide a display of the corrected and filtered intravascular image on a raster-scan display monitor. Signal processing in an intravascular ultrasound imaging system utilizing a mechanically rotated transducer operates under the assumption that the transducer is rotated at a uniform speed. However, this assumption is often violated as the catheter traverses the blood vessel. Specifically, the friction between the catheter and the vessel walls and/or the flexing of the vessel walls causes binding and/or whipping of the catheter, which translates into non-uniform rotation of the transducer. The system thus inaccurately reads the reflected echoes from blood/vessel structure/blood vessel as being received from an incorrect location, as the assumption of uniform rotational speed is violated. Therefore, it is desirable to detect and quantize the non-uniform rotation in order to correct for the image distortion caused by non-uniform rotation, and thereby provide an intravascular image display with enhanced accuracy.
Some conventional techniques used to detect non-uniform rotation of the transducer in intravascular ultrasound imaging involve calibrating the catheter 3 with landmarks or beacons 7, whether active or passive, generally located at various points (circumferentially or helically) along the perimeter of sheath 9 of the catheter 3, as seen in FIG. 1. Each beacon's position relative to the catheter is known. Passive beacons act as reflectors of ultrasound transmitted by the catheter and may undesirably cause reflective bright spots on the image which shadow points in the intravascular field behind the spots. Active beacons transmit ultrasonic energy (characterized by phase, amplitude, frequency and/or pulse repetition rate so as to identify the particular beacon) in the direction of the rotating transducer so that the imaging system may identify the particular beacon in order to determine the angular position of the transducer. However, such conventional techniques using passive or active beacons are not always effective because the beacons may cause shadowing of tissue behind the beacons or may introduce artifacts adversely affecting the imaging of the anatomical structures.
From the above, it can be seen that alternative methods and apparatus are needed for detecting non-uniform rotation distortion to allow enhanced display of intravascular ultrasound images.